Cruise Performance: Peak vs Lean of Peak

[Bob Redman]

Good morning, afternoon or evening,

I have been reading many of the posts re lean of peak operations, and considering the various arguments for & against while studying Figure 3-1 in our Lycoming Operators manual - 'Representative Effect of Fuel/Air ratio on .... and Specific Fuel Consumption at Constant RPM & Manifold Pressure'.

Our copy of Fig 3-1 illustrates that:

a. at Lycoming's 'Max Power Range' between EGTs 100 & 200 deg F Rich of Peak (ROP), percent of best power is relatively flat and at about 100% of 'Best Power'.

b. at Lycoming's 'Best Economy Range', leaned to peak EGT, percent of best power has dropped aprox 5% from 'Best Power'.

c. leaned to aprox 50 deg F Lean of Peak (LOP), percent of best power has dropped aprox 8% from 'Best Power'.

d. leaned to aprox 90 deg F LOP, percent of best power has dropped aprox 16% from 'Best power'.

e. if you lean further, then the drop in power becomes much steeper.

f. the specific fuel consumption (SFC) curve is minimum and flat from peak to 90 deg F LOP EGT, and SFC increases either side of Lycoming's 'Best Economy Range'.

Generally when I was recording cruise data for our RV-7, I chose peak EGT as a setting to maximise repeatability of the data. However, I have been experimenting with LOP cruise (below 75% max power, and no leaner than 30 deg C/ 50 deg F LOP).

I have now plotted fuel flow against observed airspeed (call it KIAS), differentiating as to whether I was at peak or LOP. The two curves are the same shape, but the LOP curve is showing:

a. about 3 litres per hour (aprox 0.8 USG/Hr) less fuel flow than the peak EGT curve at about Carson's Speed (aprox 105KIAS at 816kg/1800lb AUW), and

b. about 2 litres per hour (0.53 USG) less fuel flow at about 150 KIAS.

If valid, this data shows a clear benefit in operating LOP, eg, at 105KIAS, fuel flow is about:

a. 19 Litres per hour (5 USG/Hr) at Peak EGT, and

b. 16 Litres per hour (4.3 USG/Hr) LOP.

For comparison, at about 150 KIAS, fuel flow is about:

a. 38 Litres per hour (10 USG/Hr) at Peak EGT, and

b. 36 Litres per hour (9.5 USG/Hr) LOP.

Given the curves illustrated in Lycoming Fig 3-1, I was surprised at the result. I expected that LOP would have resulted in a lower KIAS for the same fuel flow because of the power reduction resulting from leaning below 'Max Range Power'.

The caveats on the data and result include the following:

a. the curve for peak EGT is faired to data with much less scatter than the curve for LOP data (but all LOP data is below the curve for peak EGT).

b. all data as observed on the Dynon D100 EFIS and D120 EMS - no calibration - and no correction for pressure, temp, weight, etc.

c. the data were recorded on many different days and conditions over a 18 month period, but on those days, data was either peak or LOP.

d. the pilot is now referred to as an 'older gentleman' on buses and trains, & often offered a seat by people who appear to me old enough to be my parents, but younger than my grandfather's image I see in the mirror when I am shaving, so residual competence in data collection & analysis could be an issue.

Aircraft is a standard RV-7, tip up, Aerosport IO-360M1, standard compression, ECI components, E/PMag combo, A curve advance (jumper fitted), NGK BR8EIX auto plugs, Hartzell BA constant speed, basic weight 492kg/1085lb, empty CofG 78.24 inches aft of datum, painted and complete.

I would be interested to hear if other people have noticed a similar result, and even more interested in a rational explanation for the benefit of operating LOP given my expectations based on Lycoming figure 3-1. If you use senility to explain the anomaly, I will feel aggrieved, but the offer of a fine ale would compensate my bruised ego.

Best regards

[jetdriven]

I have found that the best SFC is between 15-25 LOP on an IO-360-A3B6 engine. There is no need to lean further than that unless you have CHT problems or just want to fly slower. As you noticed the speed falls off precipitously after 50 LOP. In fact at 100 LOP the speed had fallen off a whole 15 knots from peak EGT. Absolute maximum NMPG was found at 70-80 LOP, but the speed was 10 knots slower than peak EGT, and I guess the lower drag from lower airspeed biases this figure to favor leaner mixtures and slower airspeeds. Attempting to factor this out shows that 50 LOP should be the leanest mixture setting to use for cruise, with a .2 to .4 NMPG penalty for flying at 15-25 LOP in return for 3-4 knots more airspeed. In short, the most ecomonical cost to fly a trip is found at 15-25 LOP for any power setting between 55% and 75%. Below that we just run peak EGT, the window for fuel savings before major speed loss is too small to accurately target.

On our plane, 80 ROP is 5 knots faster (155 TAS) but 1.9 GPH more fuel than 25 LOP (150 TAS). Peak EGT was .3 GPH more 1-2 knots faster than 25 LOP.

Byron
IO-360-A3B6 1977 Mooney M20J

[Bob Redman]

Thanks Byron,

You are seeing what I would expect from the Lycoming Fig 3. Thanks for the data. At the next opportunity I will look at setting MP/RPM, varying the mixture and watching KIAS, fuel flow, % power, & nm/litre.

Regards

[scsmith]

It is correct that speed should drop as you go more lean. The SFC is constant whenever LOP. So the speed loss is exactly linked to the reduction in fuel flow, since the available power is exactly linked to fuel flow.

Because power required is proportional to the velocity^3, the speed reduction should go as the cube-root of the fuel flow reduction....(this assumes constant drag coefficient, which is not really right; if you are at a speed above the best L/D speed, then as the speed drops, drag coefficient decreases) so the speed will not drop as fast as the cube-root.

The reason the SFC is constant is that when LOP, all the chemical energy is being recovered from the fuel, with no fuel wasted. There is residual oxygen, not residual fuel. Only when you lean so far that you start to misfire will you deviate from the constant SFC curve -- because the misfire wastes fuel.
There is a weak second-order effect that the available chemical energy in the fuel is related to combustion temperature, which is affected by mixture. But certainly to O(1), the SFC is constant when LOP.

[hevansrv7a]

Originally Posted by scsmith

...

The reason the SFC is constant is that when LOP, all the chemical energy is being recovered from the fuel, with no fuel wasted. There is residual oxygen, not residual fuel. Only when you lean so far that you start to misfire will you deviate from the constant SFC curve -- because the misfire wastes fuel.
There is a weak second-order effect that the available chemical energy in the fuel is related to combustion temperature, which is affected by mixture. But certainly to O(1), the SFC is constant when LOP.

This makes sense but does not agree with the data from the Advanced Pilot guys who publish actual test data from a well instrumented setup. Their data show an SFC curve against EGT very clearly.

I have not examined the data from the thread-starter, but I would expect it to show logical differences in TAS vs. fuel flow with varying mixtures. As Steve points out, the cube rule is not really accurate. At around 200 mph, the RV7 still has 10% or more of its drag from induced and at the speeds for this subject, that will be even higher.

On a related note, the SFC as reported by an EMS or EFIS - at least in my case (GRT) is highly suspect because the BHP is being calculated from MAP and RPM, etc. I can get my instrument to show me SFC's that I think are simply not realistic. That is because it is assuming my power table is correct. That internal table cannot be correct at various mixtures, only one or some that happen to agree. I admit that I don't know how other EMS/EFIS systems compute SFC, but unless they have data on torque and RPM together, their HP calculations must be based on some assumption regarding mixutre. No?

[scsmith]

But the trend with EGT is a much flatter curve than the rapid change in SFC on the ROP side, and at the extreme LOP point where misfire occurs.
That is what I meant by the secondary effect of combustion temperature.

The HP table must also include altitude, but you are right -- without knowing the mixture, it is only approximate.

We were discussing the altitude effect on a thread a while back -- it comes from the reduced back pressure on the engine at the exhaust.

[RV10inOz]

Bob,

All those answers and a whole bunch more as well as a heap of fun can be found in Brisbane and Melbourne in a few months
http://www.advancedpilot.com/livecourse-au.html

Sign up or I will send Blemish over to sort you out!

And check your email.

[jetdriven]

Originally Posted by scsmith

It is correct that speed should drop as you go more lean. The SFC is constant whenever LOP. So the speed loss is exactly linked to the reduction in fuel flow, since the available power is exactly linked to fuel flow.

Because power required is proportional to the velocity^3, the speed reduction should go as the cube-root of the fuel flow reduction....(this assumes constant drag coefficient, which is not really right; if you are at a speed above the best L/D speed, then as the speed drops, drag coefficient decreases) so the speed will not drop as fast as the cube-root.

The reason the SFC is constant is that when LOP, all the chemical energy is being recovered from the fuel, with no fuel wasted. There is residual oxygen, not residual fuel. Only when you lean so far that you start to misfire will you deviate from the constant SFC curve -- because the misfire wastes fuel.
There is a weak second-order effect that the available chemical energy in the fuel is related to combustion temperature, which is affected by mixture. But certainly to O(1), the SFC is constant when LOP.

I can't explain it, but our airplane's speed drops proportionally to fuel flow when LOP, but takes a massive loss after ~75 LOP although the fuel flow only decreased a small amount. Its pretty linear above that. BTW the engine runs smoothly to 100 LOP sometimes a bit more.

[Bob Redman]

Yesterday I tried a different approach to determine the validity of the Lycoming figure 3-1 in the original post given that, for the same KIAS, my data illustrated LOP cruise was at a lower fuel flow than cruise at peak EGT (see the original post for the caveats on the validity or otherwise of my data). The weather was not ideal for data collection - summer, MSL OAT 32 C (90F), fair weather cumulus base 3500, tops 6 to 7000, & light to occasional moderate turbulence. Military airspace above, & controlled airspace adjacent, prevented preferred higher & longer runs. I tried to hold constant manifold pressure & RPM, Dynon auto pilot engaged (alt & heading) while recording various parameters.

All data as observed on the Dynon D100 & D120: 6500 alt at 11C (52F), DA 7400, AUW at takeoff 1720lb, power set at 23/2390RPM (not full throttle). Results (in the sequence of data collection):

a. Lean of Peak (< 30C/55F), 64% power, 29 Litres/hr (7.7 USG/hr), 135KIAS.
b. Peak EGT, 76%, 35 Litres/hr (9.2 USG/hr), 142KIAS.
c. ROP (80C/144F), 41.5 Litres/hr (10.7 USG/hr), 142 KIAS.
d. Peak EGT, 75%, 35 litres/hr (9.2 USG/hr), 142 KIAS.
e. LOP (< 30C/55F), 64%, 29 Litres/hr (7.7 USG/hr), 138 KIAS.

I will try again under more favourable conditions, probably during late autumn/early winter (May/June here). Although I tried to wait for conditions to stabilise, there was a little too much turbulence for me to gather good data. Never-the-less the peak & LOP data sat within the scatter of the previous data. But LOP cruise was clearly slower than cruise at peak EGT which agrees with the Lycoming Figure 3-1, and is consistent with the comments of the other posters and my expectations. Now I suspect that the anomaly is due to plotting all (raw) data as fuel flow vs KIAS, rather than looking at still air specific fuel consumption.

I was hoping a generous flight test engineer would gently indicate the erroneous assumptions in the original post. I may be forced to review theory after a lapse of many years.

Regards,

[RV10inOz]

Bob,

Why are you doing tests other than WOT. This is the most efficient the engine will be, and at 55dF LOP this is past the point of best BSFC.

Have a closer look here.


And after the online course you will gain much as far as the best place to operate. At the moment you are dancing from the Lycoming book point to a point equally past the best BSFC.

Great to have your phone call tonight, but if this post is not making sense, call me again. You are missing the sweet spot by a small amount.

cheers!